The present invention relates to a low noise amplifier which is employed in portable cellular phones or microwave communication systems, and more particularly to a low noise amplifier which is useful for a monolithic circuit in order to prevent the functional stability from being degraded due to bonding wire for connecting ground terminals which are tied up into one node to an external circuit.
In general, a low noise amplifier is referred to as a high frequency amplifying circuit for the purpose of reducing noise parameters of entire receiver circuitry which is to be used for accepting a frequency signal with a small voltage transferred through a communication line having a long length with considerable amount of waveform loss.
As shown in FIG. 1, a typical low noise amplifier includes an input impedance circuit consisting of two capacitors 11, 12 and inductor 13, and an output impedance circuit of one capacitor 26, and also includes two MESFETs (Metal--Semiconductor Field Effect Transistors) in cascade connection instead of an interposed impedance circuit at the center stage.
The low noise amplifier introduced above has been noticed to be a reliable means with a sable operating characteristic. However, when an integrated circuit such as a microwave monolithic circuit is to be arranged for structural there is a requirement of connecting signal input/output terminals, voltage input terminals and a ground terminal to an external circuit through bonding wires These bonding wires generate some degradations in performance. Namely, an inductance along bonding wire (FIG. 2) connecting a ground terminal to a ground outside of the monolithic circuit, causes the performance of the amplifier to be no longer as stable as possible in the hybrid circuit. Reference numbers 31, 32, 33 and 34 in FIG. 2 all denote bonding wires between internal terminals and the external circuit.
These results could be explained by FIG. 3, which shows an output impedance of the first MESFET 14 plotted in Smith chart, the impedance characteristic being detected at the virtual cross sectional point taken along with the line A of FIG. 2. As shown in FIG. 3, it would be apparent that when an output signal of MESFET 14 is applied to an input of MESFET 22 the functional stability becomes reduced in accordance with the expansion of the impedance plot toward an exterior of Smith chart.
For that reason, the low noise amplifier with the conventional architecture is not adaptable to a microwave monolithic integrated circuit, just resulting in several limitations against the noise reduction and the stability enhancement. Although there has been unavoidable trends to an integration of low noise amplifier in the current of the miniaturization of portable terminal equipments, a great number of trials to control the problem described above has not found any solution for them.